Typically, ammunition magazines for firearms fall into two broad categories: fixed and detachable. Box magazines may fall into either of these categories.
Rimmed cartridges present certain challenges for designing and manufacturing reliable ammunition feeding devices, particularly for higher capacity box magazines, not encountered with rimless cartridges. The most popular types of rimmed ammunition include shotgun shells and rimfire cartridges, the latter being used in rifles and pistols.
Unlike rifle and pistol ammunition which has a very consistent standardization in length of a live cartridge from most manufacturers and/or loads, shotgun ammunition typically has significant variations in live cartridge length from one manufacturer and/or load to the next. The spectrum of length variation for rifle and pistol ammunition can typically be measured in the hundredths of an inch (0.0X0″) if not the thousands of an inch (0.00X″). The spectrum of shotgun ammunition can in some cases be measured at over an inch (X.00″) of length difference from one manufacture to the next. And more typical in the tenths of an inch (0.X00″).
Manufacturers typically call out shotgun ammunition in ¼ inch (0.25″) or ½ inch (0.5″) increments, for example, 2¾″, 3″, and 3½″. Typically, if a rifle or pistol cartridge has this much difference in length, it becomes a totally different round. For example; 9 mm Luger vs. .380 Auto, .45 Auto vs. .45 GAP, .22 Magnum vs. .22 Long Rifle vs. .22 Short, 8 mm Mauser (7.92×57) vs. 7.92×33 Kurz. Different shotgun models can typically fire everything it is chambered to (in a particular gauge) and all cartridges of shorter length. For example, the typical 12 ga shotgun that is chambered in 3″ can fire both 3″ and 2¾″ cartridges. The typical rifle or pistol model cannot do this safely and/or reliably, the few exceptions not being relevant to this discussion.
Although shotgun cartridges are typically called out in nominal ¼ inch length increments, they still greatly vary from one manufacturer and/or load to the next. Typically, the length call out is the length of the empty casing or hull. Typically, when it is loaded it loses length from crimping or rolling the casing/hull. Depending on the manufacturer and/or load, a live 2¾″ cartridge can measure less than 2.25″ in length or it can measure more than 2.55″ in length.
Another difference is that the typical shotgun ammunition is a rimmed cartridge. Most pistol and rifle ammunition has evolved into a rimless cartridge, the exceptions, again, not being relevant to this discussion. The feeding of a double stack of rimless rifle or pistol cartridges in a box magazine does not typically encounter the problem of misaligned rims.
Box magazines achieve reliability by repeatability. A typical box magazine fed weapon uses an ammunition cartridge that is very consistent in length. This prevents front to back movement and misalignment of the cartridges in the magazine under forceful movement or recoil of firing the weapon. Magazine fed weapons have an optimal position of the next cartridge to load from the top of the magazine. For example, if the cartridge is not contained or restrained to prevent longitudinal (forward and aft) movement, the round can be positioned too far forward in the magazine as it is presented for chambering, causing misalignment, or can prematurely extract from the magazine. This results in a high probability of jamming during chambering of the cartridge. For typical shotgun cartridges, it can also allow the rim of a cartridge below the top cartridge to bind the rim of the top cartridge and cause a jam (failure to feed).
A box magazine for a shotgun presents a greater challenge because of the rim and shotgun cartridges' wide range of loaded length. For this reason, there are very few box magazine fed shotguns. In the past, box magazines for shotguns have addressed this issue in only two ways, both of which include an upwardly angled stack provided by the follower. Either the cartridge was contained by holding the cartridge by its rim for the length of the magazine, preventing longitudinal movement (particularly forward movement of lower cartridges) and misalignment of cartridges and their rims. Or, it has restrained longitudinal movement (and resulting misalignment) by walls that bear against the head and forward end of the cartridge. This latter solution eliminates the use of all cartridges other than a very small portion of the spectrum of cartridge lengths. This includes many cartridges of the same nominal (call out) length. For example, not all 2¾″ cartridges would fit and/or feed reliably with this method. This is not very feasible considering the wide range of lengths of shotgun cartridges. Moreover, the shooter who is very accustomed to the versatility in the typical shotgun model to except these different lengths of cartridges does not find this acceptable.
Putting two columns of rimmed shotgun cartridges side by side makes it impossible to contain the cartridges by holding their rims in alignment. The side of the rim toward the transverse center width of the magazine (inboard side) meets the column of cartridges beside it and cannot be contained. Therefore, longitudinal movement (particularly forward movement of a lower cartridge) and misalignment and binding of rims are possible—and likely.